Just a word of warning first. There have been loads of posts from people who've bought an 'HD Ready' Freeview set and who've then been surprised to find that it can't receive Freeview HD. Please ensure that you don't make the same mistake!
If the box (or set) only has THIS logo
http://www.ksat.co.uk...ges/freeview-logo.gif
then, even though it may be 'HD Ready' it WON'T receive Freeview HD.
The 'HD Ready' bit simply means that it can display HD signals from an external source (such as a SKy box or Blu-ray player). The 'ordinary' Freeview logo tells you that the set also has a (standard definition) digital tuner built into it.
To get a set which will receive Freeview HD, you must look for THIS logo:
http://www.cnet.co.uk.../freeview_hd_logo.jpg
(Please accept my apologies if you already knew all of that!)
The simple answer to your actual question is:
Yes, a Freeview HD set will receive standard definition Freeview signals while the HD service is unavailable.
Chris